1. Field of the Invention
The present invention relates to a threshold matrix generating device and a threshold matrix generating method.
2. Description of Related Art
As for techniques for expressing quasi-gradation by a stochastic dot pattern, called FM screen, the anneal technique, the VAC (Void And Cluster) technique, the BIPPSMA technique and the DBS technique are suggested, for example. These methods, except for the DBS method, are methods for generating a threshold matrix in size of M×N pixel. A technique for actually generating a binarized image by using the created threshold matrix is the dither method. The threshold matrix created by these threshold matrix generating methods have a shape of a square or a rectangle, and is arranged in a tiled arrangement in time of screen process of the dither method to be compared to an image which is targeted for the screen process.
When the screen process is carried out by using the threshold matrix which is generated in the above exemplified methods, there is a case where lines appear in a main scanning direction or in a sub-scanning direction in the processed image. The lines appear in vicinity of the boundary of each threshold matrix when the threshold matrix of M×N pixel is arranged. For example, the lines appear when brightness (dot density) slightly differs between neighboring threshold matrices or when the dots are arranged in the same direction in the vicinity of the boundary of each threshold matrix.
Whether the unevenness such as lines occurs or not depends on how the threshold matrix is generated. Therefore, as a fundamental method for solving the problem of unevenness, there is a need to review the process of generating the threshold matrix.
In general, unevenness can easily occur when the size of the threshold matrix is small. Therefore, it is preferred to use relatively large threshold matrix of 256×256 pixels. However, there is still a possibility that unevenness occur in the vicinity of the boundaries of the threshold matrices even when the size is made to be large.
Accordingly, there is disclosed a method to prepare a plurality of threshold matrices by reversing the threshold matrices in right to left and up and down so as to use threshold matrices which are randomly selected among the plurality of threshold matrices when carrying out the screen process by the dither method to make the unevenness be obscure (for example, see JP H8-163361). Further, there is disclosed a method of carrying out the screen process by using a large size threshold matrix which is created by attaching together a plurality of types of small size threshold matrices having different threshold array (for example, see JP 2008-227924).
In the method of JP H8-163361, an object is to obtain an effect where the dot pattern obtained by the threshold matrices can be seen evenly macroscopically by rotating the threshold matrices originally having unevenness which are generated by the conventional threshold matrix generating method and by randomly combining and using the threshold matrices in which threshold arrays are changed. However, unevenness of the dot pattern itself is not being removed. Therefore, when a threshold matrix having distinct unevenness is generated, the unevenness is highly visible after all. Thus, the method of JP H8-163361 is used in a case where threshold matrix having small unevenness is generated. Further, when threshold matrices having no association are randomly attached or when bilaterally symmetric threshold matrices or threshold matrices which are symmetry about top and bottom are randomly attached, unevenness in the vicinity of the boundary of the threshold matrices may be rather greater.
In the method of JP 2008-227924, it is assumed that the small size threshold matrix is a threshold matrix which generates an even dot pattern. A dot pattern of a certain gradation is generated by screen process from the small size threshold matrix, and the small size threshold matrix is arrange in a checkerboard pattern or in a flying knight pattern so as to arrange two of them horizontally and one vertically (or vice versa), for example, and the gaps are filled with an average value of the dot pattern, and then, the Fourier transformation is performed. A bandpass filter process is carried out in the Fourier transformation side so that frequency component caused by the size of the small size threshold matrix be preferably small, and thereafter, a dot pattern is generated by carrying out an inverse Fourier transformation. From this dot pattern, a seamless large size threshold matrix can be created.
However, only the unevenness pattern which occurs in a cycle of small size threshold matrix can be removed by the above method. For example, when there is unevenness pattern that occurs in a cycle of half of the cycle of the unevenness pattern of the small size threshold matrix or that occurs in a cycle of ⅓ of the unevenness pattern of the cycle of the small size threshold matrix or the like exists in the unevenness pattern of the small size threshold matrix, those unevenness cannot be removed. Further, even when the same threshold matrices are arranged in a checkerboard pattern (in a state where the threshold matrices are arranged by being shifted by one in main scanning direction and in sub-scanning direction) or in a flying knight pattern (in a state where the threshold matrices are arrange by being shifted by one in main scanning direction and by two in sub-scanning direction), the same dot pattern is repeated every other shift or every third shift in main scanning direction or in sub-scanning direction. Therefore, the cycle of unevenness becomes larger and the unevenness can be more distinct. Thus, even in the case of the method of JP 2008-227924, it is important how the small size threshold matrix which is to be used in the beginning is generated.
Other than the above described methods, screening process may be carried out by using a threshold matrix having the same size as the image. In such case, boundaries of the threshold matrices do not exist. Therefore, there is no such problem that the above described unevenness occurring in the vicinity of the boundaries of the threshold matrices. However, when the threshold matrix is attempted to be made in a size about a regular image, a great mount of time is needed to generate the threshold matrix and further, a great amount of storage capacity is needed because the generated threshold matrix is to be stored by being saved in a storage medium such as a memory. Even if the threshold matrix can be stored in a storage medium, the calculation time of the screen process becomes long and it is not impractical.